1. Field of the Invention
The present invention concerns a method to generate an MR image of an examination subject with the use of a magnetic resonance system.
2. Description of the Prior Art
In conventional imaging with a magnetic resonance system (MRI—“magnetic resonance imaging”), certain image artifacts are caused by excited spins that are located outside of the prescribed field of view (FOV—“field of view”). These artifacts occur particularly in MR imaging methods or techniques that use a spatially non-selective excitation or a slice-selective excitation, insofar as the dimensions of the examination subject orthogonal to the slice selection direction (vector situated orthogonally to the slice) exceed the prescribed dimensions of the field of view. With a radial scanning of k-space, these artifacts occur as interfering stripes outside of a disc or sphere around the excited subject. For example, when MR images of a human head are generated, these stripes are caused by a high intensity spin enrichment (accumulation) outside of the field of view in the region of the neck of the patient, which is detected by the coil elements that are located in proximity to the neck region.
The imaging properties in MR imaging can be described with the use of the point function or point spread function (PSF), which has a direct relationship to the scan patterns that are used in order to acquire information in k-space. Mathematically, the data acquisition along a trajectory in k-space can be considered as a projection in the sense of a multiplication of k-space on the trajectory that corresponds to a convolution of the subject by means of a Fourier transformation of the trajectory in image space. The imaging method can therefore be described in image space by a folding of the excited spins by means of the Fourier transformation of the trajectory or by means of the point response.
In radial scanning of k-space, wherein the information in k-space is acquired along defined spokes, the point function has a peak value in the middle, which is surrounded by a nearly homogenous disc (in two-dimensional scanning) or a nearly homogenous sphere (in three-dimensional scanning) with a negligible intensity. At a defined distance from the peak value, the point function has radial stripes that proceed outwardly (toward the edge) from the disc (or the sphere). The distance is also known as a Nyquist radius. The distance becomes larger as the number of acquired spokes increases. The distance is therefore used to define the number of required spokes so that the image of the corresponding subject can be generated without visible stripe-shaped artifacts.
In practice, due to the limited available acquisition time, it is conventionally not possible to acquire sufficiently many spokes in order to satisfy this criterion for all dimensions. Therefore, excited spins that are located outside of the prescribed field of view can have a perturbing effect on the middle segment of the region of interest (ROI) if their intensity is high enough. This is a problem particularly in the use of saturated techniques (for example fat-saturation methods), which generally are no longer effective at a defined distance from the isocenter, because points with a high intensity occur at locations at the edge. This problem is intensified if reception coils or reception coil elements (“array coils”) with a high sensitivity are used outside of the center, which leads to high signal peak signal values, so a large proportion of this intensity extends as radial stripes across the entire image. Moreover, excited spins (which are located at the edge of the field of view along the z-direction in which the gradient fields no longer have a linear relationship) lead to significant perturbations in the image.
These described image artifacts can lead to difficult problems, both for the observer of the images and for automatic, downstream methods (for example segmentation methods). It must be taken into account that precisely such segmentation methods are very sensitive to stripe-shaped artifacts. An important example in this regard is the acquisition and automatic post-processing (segmentation) of images that are generated with a radial, ultrashort sequence (UTE—“ultrashort echo time”), wherein an MR-based attenuation correction with a hybrid PET-MR imaging is used.